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   <title>SRFI 5: A compatible let form with signatures and rest arguments</title>
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<H1>Title</H1>

SRFI-5: A compatible <code>let</code> form with signatures and rest arguments

<H1>Author</H1>

Andy Gaynor

<H1>Status</H1>

This SRFI is currently in ``final'' status.  To see an explanation of each status that a SRFI can hold, see <A HREF="http://srfi.schemers.org/srfi-process.html">here</A>.
You can access the discussion on this SRFI via <A HREF="http://srfi.schemers.org/srfi-5/mail-archive/maillist.html">the archive of the mailing list</A>.
<P><UL>
<LI>Received: 1999/2/2
<LI>Draft: 1999/2/10-1999/04/12
<LI>Final: 1999/4/26
<LI>Revised reference implementation: 2003/01/27
</UL>

<H1>Abstract</H1>

The <i>named-let</i> incarnation of the <code>let</code> form has two slight
inconsistencies with the <code>define</code> form.  As defined, the <code>let</code>
form makes no accommodation for rest arguments, an issue of functionality
and consistency.  As defined, the <code>let</code> form does not accommodate
signature-style syntax, an issue of aesthetics and consistency.  Both
issues are addressed here in a manner which is compatible with the traditional
<code>let</code> form but for minor extensions.

<H1>Rationale</H1>

<H2>Signature-style Syntax</H2>

Consider the following two equivalent definitions:

<p><pre>
(define fibonacci
  (lambda (n i f0 f1)
    (if (= i n)
      f0
      (fibonacci n (+ i 1) f1 (+ f0 f1)))))

(define (fibonacci n i f0 f1)
  (if (= i n)
    f0
    (fibonacci n (+ i 1) f1 (+ f0 f1))))
</pre>

Although there is a named-let analog for the former form, there is none
for the latter.  To wit, suppose one wished to compute the 10th element
of the Fibonacci sequence using a named let:

<p>
<pre>
(let fibonacci ((n 10) (i 0) (f0 0) (f1 1))
  (if (= i n)
    f0
    (fibonacci n (+ i 1) f1 (+ f0 f1))))
Values: 55
</pre>

As it stands, one cannot equivalently write

<p>
<pre>
(let (fibonacci (n 10) (i 0) (f0 0) (f1 1))
  (if (= i n)
    f0
    (fibonacci n (+ i 1) f1 (+ f0 f1))))
</pre>

which is consistent with <code>define</code>'s signature-style form.
<p>Those that favor the signature style may prefer this extension. 
In any case, it may be more appropriate to include all bound names within
the binding section.  As presented, this straightforward extension
introduces no ambiguity or incompatibility with the existing definition
of let.

<H2>Rest Arguments</H2>

As it stands, one cannot write a named let with rest arguments, as in

<p>
<pre>
(let (blast (port (current-output-port)) . (x (+ 1 2) 4 5))
  (if (null? x)
    'just-a-silly-contrived-example
    (begin
      (write (car x) port)
      (apply blast port (cdr x)))))
</pre>

otherwise equivalent to

<p>
<pre>
(letrec ((blast (lambda (port . x)
		  (if (null? x)
		      'just-a-silly-contrived-example
		      (begin
			(write (car x) port)
			(apply blast port (cdr x)))))))
  (blast (current-output-port) (+ 1 2) 4 5))
</pre>

While this example is rather contrived, the functionality is not. 
There are several times when the author has used this construct in practice. 
Regardless, there is little reason to deny the <code>let</code> form access to
all the features of lambda functionality.

<H2>Symbols in Binding Sections</H2>

Both the features above rely upon the placement of symbols in <code>let</code>
binding lists (this statement is intentially simplistic).  The only
other apparent use of such symbol placement is to tersely bind variables
to unspecified values.  For example, one might desire to use
<code>(let (foo bar baz) ...)</code>
to bind <code>foo</code>, <code>bar</code>, and <code>baz</code> to
unspecified values.

<p>This usage is considered less important in light of the rationales
presented above, and an alternate syntax is immediately apparent, as
in <code>(let ((foo) (bar) (baz)) ...)</code> This may even
be preferable, consistently parenthesizing normal binding clauses.

<H1>Specification</H1>

<H2>Syntax</H2>

<p>
A formal specification of the syntax follows.  Below, body, expression,
and identifier are free.  Each instantiation of binding-name must be
unique.
</p>

<p>
<pre>
            let = "(" "let" let-bindings body ")"
    expressions = nothing | expression expressions
   let-bindings = let-name bindings
                | "(" let-name "." bindings ")"
       let-name = identifier
       bindings = "(" ")"
                | rest-binding
                | "(" normal-bindings ["." rest-binding] ")"
normal-bindings = nothing
                | normal-binding normal-bindings
 normal-binding = "(" binding-name expression ")"
   binding-name = identifier
   rest-binding = "(" binding-name expressions ")"
</pre>

<p>
For clarity and convenience, an informal specification follows.
</p>

<ol>
<li><a name="unnamed">Unnamed</a>

<p><pre>
(let ((&lt;parameter&gt; &lt;argument&gt;)...) 
  &lt;body&gt;...)
</pre>
</li>

<li><a name="named-non-sig">
Named, non-signature-style, no rest argument</a>

<p><pre>
(let &lt;name&gt; ((&lt;parameter&gt; &lt;argument&gt;)...)
  &lt;body&gt;...)
</pre>
</li>

<li><a name="named-sig">Named, signature-style,  no rest argument</a>

<p><pre>
(let (&lt;name&gt; (&lt;parameter&gt; &lt;argument&gt;)...)
  &lt;body&gt;...)
</pre>
</li>

<li><a name="named-non-sig-rest">Named, non-signature-style, rest argument</a>

<p><pre>
(let &lt;name&gt; ((&lt;parameter&gt; &lt;argument&gt;)...
    
. (&lt;rest-parameter&gt; &lt;rest-argument&gt;...))
  &lt;body&gt;...)
</pre>

<li><a name="named-sig-rest">Named, signature-style, rest argument</a>

<p><pre>
(let (&lt;name&gt; (&lt;parameter&gt; &lt;argument&gt;)...
   
. (&lt;rest-parameter&gt; &lt;rest-argument&gt;...))
  &lt;body&gt;...)
</pre>
</li>
</ol>

<H2>Semantics</H2>

Let <code>$lambda</code> and <code>$letrec</code> be hygienic bindings for the <code>lambda</code>
and <code>letrec</code> forms, respectively.

<ul>
<li>For informal syntax 1:

<p><pre>
(($lambda (&lt;parameter&gt;...) &lt;body&gt;...) &lt;argument&gt;...)
</pre>
</li>

<li>For informal syntaxes 2 and 3:

<p>
<pre>
($letrec ((&lt;name&gt; ($lambda (&lt;parameter&gt;...) &lt;body&gt;...)))
  (&lt;name&gt; &lt;argument&gt;...))
</pre>
</li>

<li>For informal syntaxes 4 and 5:

<p>
<pre>
($letrec ((&lt;name&gt; ($lambda (&lt;parameter&gt;...
   
. &lt;rest-parameter&gt;) &lt;body&gt;...))) 
  (&lt;name&gt; &lt;argument&gt;... &lt;rest-argument&gt;...))
</pre>
</li>
</ul>

<H1>Implementation</H1>

Here is an implementation using <code>SYNTAX-RULES</code>.

<p>
<pre>
;; Use your own standard let.
;; Or call a lambda.
;; (define-syntax standard-let
;;
;;   (syntax-rules ()
;;
;;     ((let ((var val) ...) body ...)
;;      ((lambda (var ...) body ...) val ...))))

(define-syntax let

  (syntax-rules ()

    ;; No bindings: use standard-let.
    ((let () body ...)
     (standard-let () body ...))
    ;; Or call a lambda.
    ;; ((lambda () body ...))

    ;; All standard bindings: use standard-let.
    ((let ((var val) ...) body ...)
     (standard-let ((var val) ...) body ...))
    ;; Or call a lambda.
    ;; ((lambda (var ...) body ...) val ...)

    ;; One standard binding: loop.
    ;; The all-standard-bindings clause didn't match,
    ;; so there must be a rest binding.
    ((let ((var val) . bindings) body ...)
     (let-loop #f bindings (var) (val) (body ...)))

    ;; Signature-style name: loop.
    ((let (name binding ...) body ...)
     (let-loop name (binding ...) () () (body ...)))

    ;; defun-style name: loop.
    ((let name bindings body ...)
     (let-loop name bindings () () (body ...)))))

(define-syntax let-loop

  (syntax-rules ()

    ;; Standard binding: destructure and loop.
    ((let-loop name ((var0 val0) binding ...) (var ...     ) (val ...     ) body)
     (let-loop name (            binding ...) (var ... var0) (val ... val0) body))

    ;; Rest binding, no name: use standard-let, listing the rest values.
    ;; Because of let's first clause, there is no "no bindings, no name" clause.
    ((let-loop #f (rest-var rest-val ...) (var ...) (val ...) body)
     (standard-let ((var val) ... (rest-var (list rest-val ...))) . body))
    ;; Or call a lambda with a rest parameter on all values.
    ;; ((lambda (var ... . rest-var) . body) val ... rest-val ...))
    ;; Or use one of several other reasonable alternatives.

    ;; No bindings, name: call a letrec'ed lambda.
    ((let-loop name () (var ...) (val ...) body)
     ((letrec ((name (lambda (var ...) . body)))
        name)
      val ...))

    ;; Rest binding, name: call a letrec'ed lambda.
    ((let-loop name (rest-var rest-val ...) (var ...) (val ...) body)
     ((letrec ((name (lambda (var ... . rest-var) . body)))
        name)
      val ... rest-val ...))))
</pre>


<H1>Copyright</H1>

Copyright (C) Andy Gaynor (1999). All Rights Reserved.
<p>This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and distributed,
in whole or in part, without restriction of any kind, provided that the
above copyright notice and this paragraph are included on all such copies
and derivative works. However, this document itself may not be modified
in any way, such as by removing the copyright notice or references to the
Scheme Request For Implementation process or editors, except as needed
for the purpose of developing SRFIs in which case the procedures for copyrights
defined in the SRFI process must be followed, or as required to translate
it into languages other than English.
<p>The limited permissions granted above are perpetual and will not be
revoked by the authors or their successors or assigns.
<p>This document and the information contained herein is provided on an
"AS IS" basis and THE AUTHOR AND THE SRFI EDITORS DISCLAIM ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE
USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    <hr>
    <address>Editor: <a href="mailto:srfi-editors@srfi.schemers.org">Mike Sperber</a></address>

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